Characterization of the receptor-destroying enzyme activity from infectious salmon anaemia virus

Structures of human-infecting Thogotovirus fusogens support a common ancestor with insect baculovirus

Thogotoviruses are emerging tick-borne zoonotic orthomyxoviruses infecting both humans and domestic animals with severe clinical consequences. These viruses utilize a single-envelope glycoprotein (Gp) to facilitate their entry into host cells. Here, we present the Gp structures of Thogoto and Dhori viruses, both of which are members of the Thogotovirus genus in the family Orthomyxoviridae These structures, determined in the postfusion conformation, identified them as class III viral fusion proteins. It is intriguing that the Gp structures are similar to the envelope protein of baculovirus, although sharing a low sequence identity of ∼28%. Detailed structural and phylogenic analyses demonstrated that these Gps originated from a common ancestor. Among the structures, domain I is the most conserved region, particularly the fusion loops. Domain II showed the highest variability among different viruses, which might be related to their distinct host tropism. These findings increase our understanding of the divergent evolution processes of various orthomyxoviruses and indicate potential targets for developing antiviral therapeutics by intercepting virus entry.

Orthomyxoviruses of Fish

The family Orthomyxoviridae is well known for containing influenza viruses with a segmented RNA genome that is prone to gene reassortment in mixed infections (known as antigenic shift) resulting in new virus subtypes that cause pandemics, and cumulative mutations (known as antigenic drift), resulting in new virus strains that cause epidemics. This family also contains infectious salmon anemia virus (ISAV) and tilapia lake virus (TiLV), which are a unique orthomyxoviruses that infect fish and is unable to replicate above room temperature (24°C). This chapter describes the comparative virology of members in the family Orthomyxoviridae in general, helping to understand the emergent teleost orthomyxoviruses, ISAV and TiLV. The most current information on virus–host interactions of the fish orthomyxoviruses, particularly ISAV, as they relate to variations in virus structure, virulence, persistence, host range and immunological aspects is presented in detail.

An optoelectrokinetic technique for programmable particle manipulation and bead-based biosignal enhancement

Technologies that can enable concentration of low-abundance biomarkers are essential for early diagnosis of diseases. In this study, an optoelectrokinetic technique, termed Rapid Electrokinetic Patterning (REP), was used to enable dynamic particle manipulation in bead-based bioassays. Various manipulation capabilities, such as micro/nanoparticle aggregation, translation, sorting and patterning, were developed. The technique allows for versatile multi-parameter (voltage, light intensity and frequency) based modulation and dynamically addressable manipulation with simple device fabrication. Signal enhancement of a bead-based bioassay was demonstrated using dilute biotin-fluorescein isothiocyanate (FITC) solutions mixed with streptavidin-conjugated particles and rapidly concentrated with the technique. As compared with a conventional ELISA reader, the REP-enabled detection achieved a minimal readout of 3.87 nM, which was a 100-fold improvement in sensitivity. The multi-functional platform provides an effective measure to enhance detection levels in more bead-based bioassays.

Sialic Acid Receptors of Viruses

Sialic acid linked to glycoproteins and gangliosides is used by many viruses as a receptor for cell entry. These viruses include important human and animal pathogens, such as influenza, parainfluenza, mumps, corona, noro, rota, and DNA tumor viruses. Attachment to sialic acid is mediated by receptor binding proteins that are constituents of viral envelopes or exposed at the surface of non-enveloped viruses. Some of these viruses are also equipped with a neuraminidase or a sialyl-O-acetyl-esterase. These receptor-destroying enzymes promote virus release from infected cells and neutralize sialic acid-containing soluble proteins interfering with cell surface binding of the virus. Variations in the receptor specificity are important determinants for host range, tissue tropism, pathogenicity, and transmissibility of these viruses.

Immune responses in Atlantic salmon (Salmo salar) following protective vaccination against infectious salmon anemia (ISA) and subsequent ISA virus infection

Infectious salmon anemia (ISA) is an orthomyxoviral disease that has had devastating effects on farmed Atlantic salmon. ISA is still a disease resulting in continued loss of revenues and therefore development of effective vaccines is of great importance. Commercial vaccines against ISA are available, but the efficacy is poorly described. There is little information about vaccine-induced immune factors preventing ISA virus (ISAV) infection today. In this study we assessed the protective effects and immunogenicity of vaccines containing three different quantities of the inactivated ISAV antigen. Our findings indicated that immunization induced effective protection in Atlantic salmon with a relative percent survival (RPS) as high as 86. The level of protection was correlated to the amount of ISAV antigen in the vaccine, and fish immunized with high antigen amounts produced detectable ISAV-specific and neutralizing antibodies. While ISAV infection was detectable in non-vaccinated control fish challenged by cohabitation, no infection was detected in fish immunized with high antigen amounts. After challenge, transcriptional analysis of selected immune-related genes demonstrated activation of innate immune responses in ISAV-infected control fish, but not in vaccine protected fish. This study furthers the knowledge about vaccine efficacy and vaccine-induced immunity to ISAV challenge in Atlantic salmon.

Completion of the full-length genome sequence of the infectious salmon anemia virus, an aquatic orthomyxovirus-like, and characterization of mAbs

We report here the first full-length sequence of the eight ssRNA genome segments of the infectious salmon anemia virus (ISAV, Glesvaer/2/90 isolate), a salmonid orthomyxovirus-like. Comparison of ISAV genome sequence with those of others orthomyxovirus reveals low identity values, and a remarkable feature is the extremely long 5' end UTR of ISAV segments, which all contain an additional conserved motif of unknown function. In addition to the genome nucleotide sequence determination, specific mAbs have been produced through mice immunization with sucrose-purified ISAV. Four mAbs directed against the haemagglutinin-esterase glycoprotein, the nucleoprotein and free or actin-associated forms of the matrix protein have been characterized by (i) indirect fluorescent antibody test; (ii) virus neutralization; (iii) radioimmunoprecipitation and (iv) Western blot assays. These mAbs will potentially be useful for the development of new diagnostic tests, and the nucleotide sequences will help to establish a reverse genetics system for ISAV.

Infectious salmon anemia virus--genetics and pathogenesis

The infectious salmon anemia virus (ISAV) is the causative agent of the ISA syndrome that affects mainly Atlantic salmon (Salmo salar) and has caused high mortality epidemics in Norway, Scotland, Canada, the United States and Chile. It is classified as an Orthomyxoviridae, its genome is composed of 8 single-strand RNA segments with negative polarity that code for 11 polypeptides. Through functional studies of the coded proteins it has been established that RNA segments 5 and 6 code for a fusion protein and hemagglutinin, respectively, while two polypeptides coded by segments 7 and 8 inhibit interferon induction. The functions of the rest of the possible proteins coded by the viral genome have been assigned by comparison with the corresponding ones of the influenza virus genome. As to its pathogenicity, some growth parameters such as incubation period, resistance to chemical and physical factors, establishment of the infection in other marine species, and dissemination ability among the different organs have been evaluated in several salmonids. Genomic analysis has shown (i) the existence of a high polymorphism region (HPR) in segment 6, and (ii) sequence insertion in segment 5. More than 20 HPR variants have been determined, all originating from HPR0, which is associated with low pathogenicity, while 4 different sequence insertions in segment 5 have not been related with some characteristic of the virus infection. Much progress has been made in the characterization of the virus in 20 years of study, but more detailed knowledge of the specific function of the proteins coded by all the viral genes is still missing, including the pathogenicity mechanism at the molecular level.

Bioinformatic analysis of the genome of infectious salmon anemia virus associated with outbreaks with high mortality in Chile

The infectious salmon anemia virus (ISAV), an orthomyxovirus, is the major cause of outbreaks of high mortality rates in salmon in Chile. It has been proposed that the virulence of ISAV isolates lies mainly in hemagglutinin-esterase and fusion glycoproteins. However, based on current information, the contribution of other viral genes cannot be ruled out. To study this, we isolated and determined the complete coding sequence of two high-prevalence Chilean isolates associated with outbreaks of high mortality rates: ISAV752_09 and ISAV901_09. These isolates were compared to 15 Norwegian isolates that exhibit differences in their virulence. For this purpose, we performed bioinformatic analyses of (i) functional domains, (ii) specific mutations, (iii) Bayesian phylogenetics, and (iv) structural comparisons between ISAV and influenza virus glycoproteins by using molecular modeling. Phylogenetic analysis shows two genogroups for each protein, one of them containing the Chilean isolates. The gene sequence of the polymerase complex and nucleoprotein indicated that they are closely related to homologues from highly pathogenic Norwegian viruses. Notably, seven of the eight mutations that are present only in the Chilean isolates are on the polymerase complex and nucleoprotein. Structural modeling of hemagglutinin-esterase shows patches of variable residues on its surface. Fusion protein modeling shows that insertions are flexible regions that could affect proteolytic processing, increasing either the accessibility or the number of recognition sites for specific proteases. We found antigenic drift processes related to insertion into the isolated segment 5 of the ISAV752_09. Our results confirm the European origin of Chilean isolates to be the result of reassortments from Norwegian ancestors.

Structural and functional analysis of the hemagglutinin-esterase of infectious salmon anaemia virus

Infectious salmon anaemia virus (ISAV) is a piscine orthomyxovirus causing a serious disease in farmed Atlantic salmon (Salmo salar L.). The virus surface glycoprotein hemagglutinin-esterase (HE) is responsible for both viral attachment and release. Similarity to bovine and porcine torovirus hemagglutinin-esterase (BToV HE, PToV HE), bovine coronavirus HE (BCoV HE) and influenza C hemagglutinin-esterase-fusion (InfC HEF) proteins were exploited in a computational homology-based structure analysis of ISAV HE. The analysis resolved structural aspects of the protein and identified important features of relevance to ISAV HE activity. By recombinant expression and purification of secretory HE (recHE) proteins, receptor-binding and quantitative analyses of enzymatic activities displayed by ISAV HE molecules are presented for the first time. Three different recHE molecules were constructed: one representing a high virulent isolate, one a low virulent, while in the third a Ser(32) to Ala(32) amino acid substitution was introduced in the enzymatic catalytic site as inferred from the model. The three amino acid differences between the high and low virulent variants, of which two localized to the putative receptor-binding domain and one in the esterase domain, had no impact on receptor-binding or -release activities. In contrast, the Ser(32) amino acid substitution totally abolished enzymatic activity while receptor binding increased, as observed by agglutination of Atlantic salmon red blood cells. This demonstrates the essential role of a serine in the enzyme's catalytic site. In conclusion, structural analysis of ISAV HE in combination with selected recHE proteins gave insights into structure-function relationships and opens up for further studies aiming at dissecting molecular determinants of ISAV virulence.

Parasitic crustaceans as vectors of viruses, with an emphasis on three penaeid viruses

Parasitic crustaceans serve as both hosts and vectors of viruses as well as of parasites and other microbial pathogenic agents. Few of the presumably numerous associations are known, but many can be anticipated. Recently, branchiurans and gnathiid isopods have been documented to host helminths and blood parasites. Because the agents can be observed readily with a microscope, these are better recognized than are the smaller viral, bacterial, and fungal agents. Some agents are harmful to the host of the crustacean parasite and others are not. Viruses probably fit both these categories, since viruses that do not appear pathogenic are often seen in ultrastructural images from a range of invertebrate hosts, including crustaceans. Some viruses have been implicated in causing disease in the host, at least under appropriate conditions. For example, lymphocystis virus may possibly be transmitted to the dermis of its fish hosts by copepods and to the visceral organs by a cymothoid isopod. Similarly, argulid branchiurans seem to transmit the viral agent of spring viremia of carp as well as carp pox, and copepods have been implicated in transmitting infectious hematopoietic necrosis, infectious salmon anemia, and infectious pancreatic necrosis to salmon. Other viruses can be vectored to their hosts through an additional animal. We exposed three viruses, Taura syndrome virus (TSV), white spot syndrome virus (WSSV), and yellowhead virus (YHV), all of which cause mortalities in wild and cultured penaeid shrimps, to crustacean parasites on fish and crabs. Using real-time polymerase chain reaction analysis, we show that TSV in the cyclopoid copepod Ergasilus manicatus on the gill filaments of the Gulf killifish, Fundulus grandis, the acorn barnacle Chelonibia patula on the carapace of the blue crab, Callinectes sapidus, and gooseneck barnacle Octolasmis muelleri on the gills of C. sapidus, can replicate for at least 2 weeks and establish what should be an infective dose. This result was additionally supported by positive in situ hybridization reactions. All three parasites are the first known non-penaeid hosts in which replication occurs. The mean log copy number of WSSV also suggested that replication occurred in E. manicatus. The mean log copy number of YHV gradually decreased in all three parasites and both hosts over the 2-week period. The vector relationships indicate an additional potential means of transmitting and disseminating the disease-causing agents to the highly susceptible and economically valuable penaeid shrimp hosts.

Comparative virulence of Infectious salmon anaemia virus isolates in Atlantic salmon, Salmo salar L

Infectious salmon anaemia virus (ISAV) surveillance in the Bay of Fundy has identified the existence of a large number of genetically distinct ISAV isolates which appear to be of variable virulence. Genetically distinct isolates are currently being designated based on sequencing of the hyper polymorphic region (HPR) of genomic segment 6, which encodes the haemagglutinin-esterase protein, but it has been difficult to elucidate a clear association between these molecular variations and variations in virulence. This has hampered the establishment of proactive management decisions regarding infected fish, and ISAV infections, regardless of type, must be treated as one. Field data of ISAV infections is difficult to collect and to compare between infections because of a wide range of confounding factors including time of year, fish stock, cage site location, mitigating factors and stressors. An important tool in determining the relationship between molecular differences and virulence comes from analysis of quarantine studies. The goal of this study was to compare the virulence, by co-habitation and intraperitoneal injection, of four regionally common and recent ISAV isolates in a controlled environment. We found significant differences in mortality between ISAV molecular isolates, and present data showing that survival of ISAV infection confers significant resistance to re-infection with a different ISAV isolate. These findings, if borne out in field studies, will significantly alter the way ISAV infections are managed in the Bay of Fundy and elsewhere.

Heterologous expression and purification of the infectious salmon anemia virus hemagglutinin esterase

This study presents the heterologous production and purification of a soluble and functional form of the hemagglutinin esterase (HE) of the infectious salmon anemia virus (ISAV) isolate 4 (Glesvaer/2/90). The HE possesses receptor binding and receptor destroying enzyme (RDE) activity and is probably involved in the infection process. The recombinant HE protein (recHE 4) was expressed in insect cells (Sf9) using the baculovirus expression vector system. Both the transmembrane region and the cytoplasmic tail were deleted, and a C-terminal His(6)-tag was attached to facilitate identification and purification of the recHE 4 protein. As determined by Western analysis the recHE 4 was secreted at 20 degrees C and not at 28 degrees C. By testing three HE constructs differing in their promoter and secretion signal sequences it was clear that the HE's own secretion signal sequence is more important than the promoter with respect to the amount of secreted recHE 4 obtained under the conditions used. A one-step purification by nickel-affinity chromatography resulted in a highly purified recHE 4, identified by matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) analysis. Also, the recHE 4 is glycosylated and contains disulfide bridges within the molecule. Functional studies including the verification of the receptor destroying enzyme (RDE) activity as well as the binding to Atlantic salmon erythrocytes (hemagglutination) indicate that the recHE 4 has similar functions as its native counterpart. In conclusion, insect cells secrete a functional form of the ISAV 4 HE. This is suitable for further analyses on its function and immunogenicity.

Gene expression analyses in Atlantic salmon challenged with infectious salmon anemia virus reveal differences between individuals with early, intermediate and late mortality

BACKGROUND

Infectious salmon anemia virus (ISAV) causes a multisystemic disease responsible for severe losses in salmon aquaculture. Better understanding of factors that explain variations in resistance between individuals and families is essential for development of strategies for disease control. To approach this, we compared global gene expression using microarrays in fish dying early and late in the time course following infection from a highly pathogenic ISAV.

RESULTS

Tissues (gill, heart, liver and spleen) from infected Atlantic salmon (cohabitation, ISAV Glesvaer 2/90 isolate) were collected from three stages over the time course of the experiment; early (EM, 0-10% cumulative mortality (CM), 21-25 days post-infection (DPI)), intermediate (IM, 35-55% CM, 28-31 DPI) and late (LM, 75-85% CM, 37-48 DPI) mortality. Viral loads were equal in EM and IM but dropped markedly in LM fish. Gene expression analyses using a 1.8 K salmonid fish cDNA microarray (SFA2.0) and real-time qPCR revealed a large group of genes highly up-regulated across tissues in EM, which were mainly implicated in innate antiviral responses and cellular stress. Despite equal levels of MHC class I in EM and LM, increase of splenic and cardiac expression of immunoglobulin-like genes was found only in LM while a suite of adaptive immunity markers were activated already in IM. The hepatic responses to ISAV were characterized by difference between EM and LM in expression of chaperones and genes involved in eicosanoid metabolism. To develop classification of high and low resistance phenotypes based on a small number of genes, we processed results from qPCR analyses of liver using a linear discriminant analysis. Four genes (5-lipoxygenase activating protein, cytochrome P450 2K4-1, galectin-9 and annexin A1) were sufficient for correct assignment of individuals to EM, LM and uninfected groups, while IM was inseparable from EM. Three of four prognostic markers are involved in metabolism of inflammatory regulators.

CONCLUSION

This study adds to the understanding of molecular determinants for resistance to acute ISAV infection. The most susceptible individuals were characterized by high viral replication and dramatic activation of innate immune responses, which did not provide protection. The ability to endure high levels of infection for sustained periods could be associated with lower inflammatory responses while subsequent protection and viral clearance was most likely conferred by activation of adaptive immunity.

Characterization of Chinook head salmon embryo phenotypes of infectious salmon anemia virus by real-time RT-PCR

We have previously described the development of a one-tube SYBR Green real-time RT-PCR assay for the detection and quantitation of infectious salmon anemia virus (ISAV) in various biological samples. The twofold aim of the present study was to verify that the optimized SYBR Green real-time RT-PCR conditions could detect ISAV isolates of different geographic origins, and to analyze the growth patterns of the selected ISAV isolates in the Chinook head salmon embryo (CHSE)-214 cells by this assay to better characterize their CHSE-phenotypes. A total of 24 ISAV isolates were used in this study. The results indicated that the SYBR Green real-time RT-PCR could detect ISAV of different geographic origins or laboratory sources. The capacity of ISAV isolates to cause cytopathic effects (CPE) in the CHSE-214 cell line, viral titration of the infected CHSE-cell harvests, and analysis of viral RNA levels in CHSE-214 cells at post-infection day zero, 7 and 14 by SYBR Green real-time RT-PCR confirmed the existence of three CHSE-phenotypes of ISAV: replicating cytopathic, replicating non-cytopathic, and non-replicating non-cytopathic. The identification of these three CHSE-phenotypes of ISAV has important implications from diagnostic and biological points of view.

Characterization of the infectious salmon anemia virus fusion protein

Infectious salmon anemia virus (ISAV) is an orthomyxovirus causing serious disease in Atlantic salmon (Salmo salar L.). This study presents the characterization of the ISAV 50-kDa glycoprotein encoded by segment 5, here termed the viral membrane fusion protein (F). This is the first description of a separate orthomyxovirus F protein, and to our knowledge, the first pH-dependent separate viral F protein described. The ISAV F protein is synthesized as a precursor protein, F0, that is proteolytically cleaved to F1 and F2, which are held together by disulfide bridges. The cleaved protein is in a metastable, fusion-activated state that can be triggered by low pH, high temperature, or a high concentration of urea. Cell-cell fusion can be initiated by treatment with trypsin and low pH of ISAV-infected cells and of transfected cells expressing F, although the coexpression of ISAV HE significantly improves fusion. Fusion is initiated at pH 5.4 to 5.6, and the fusion process is coincident with the trimerization of the F protein, or most likely a stabilization of the trimer, suggesting that it represents the formation of the fusogenic structure. Exposure to trypsin and a low pH prior to infection inactivated the virus, demonstrating the nonreversibility of this conformational change. Sequence analyses identified a potential coiled coil and a fusion peptide. Size estimates of F1 and F2 and the localization of the putative fusion peptide and theoretical trypsin cleavage sites suggest that the proteolytic cleavage site is after residue K276 in the protein sequence.

Characterization ofN- andO-glycopeptides of recombinant human erythropoietins as potential biomarkers for doping analysis by means of microscale sample purification combined with MALDI-TOF and quadrupole IT/RTOF mass spectrometry

The structural characterization of the O- and N-glycan structures of three different commercially available recombinant human erythropoietins (rhEPOs) is represented by means of a microscale sample purification using ZipTip technology and MALDI-TOF and MALDI low-energy CID MS. Glycopeptides were released from rhEPO samples by a differential endoproteolytic digestion to obtain site-specific glycosylation patterns. Mass accuracies in the range of +/- 0.04% obtained by the high-resolution TOF instrument allowed an unambiguous assignment of N-glycan structures via glycan database software. Furthermore, the O-glycan structures were directly analyzed on the glycopeptide level by MS/MS experiments. Principally, site-specific glycosylation was found to be very similar for the three different rhEPOs (EPO-alpha, EPO-beta, and novel erythropoiesis stimulating protein (NESP)) but exhibiting quantitative differences in distinct O- and N-glycan moieties. Significant differences were found in the degree of sialylation and acetylation. Especially, a considerable degree of variation of the O-acetylation of sialic acid residues could be realized on the glycan structures of O- and N-glycopeptides, whereas EPO-alpha and EPO-beta could be clearly differentiated from NESP solely on the O-glycopeptide level.

Infectious salmon anemia virus specifically binds to and hydrolyzes 4-O-acetylated sialic acids

Infectious salmon anemia virus (ISAV) is the causative agent of infections in farmed Atlantic salmon. ISAV presumably represents a new genus within the Orthomyxoviridae. ISAV has been shown earlier to exhibit a receptor-destroying activity, which was defined as an acetylesterase with unknown specificity. We have analyzed the substrate specificity of the ISAV esterase in detail. Purified ISAV hydrolyzed free 5-N-acetyl-4-O-acetyl neuraminic acid. In addition, the purified 9-O-acetylated sialic acid derivative was also hydrolyzed, but at lower rates. When we used a glycosidically bound substrate, ISAV was unable to hydrolyze 9-O-acetylated sialic acid, which represents the major substrate for the influenza C virus esterase. ISAV completely de-O-acetylated glycoprotein-bound 5-N-acetyl-4-O-acetyl neuraminic acid. Thus, the enzymatic activity of the hemagglutinin-esterase of ISAV is comparable to that of the sialate-4-O-esterases of murine coronaviruses and related group 2 coronaviruses. In addition, we found that ISAV specifically binds to glycoproteins containing 4-O-acetylated sialic acids. Both the ISAV esterase and recombinant rat coronavirus esterase specific for 4-O-acetylated sialic acids hydrolyzed ISAV receptors on horse and rabbit erythrocytes, indicating that this sialic acid represents a receptor determinant for ISAV.

Recombinant viral sialate-O-acetylesterases

Viral O-acetylesterases were first identified in several viruses, including influenza C viruses and coronaviruses. These enzymes are capable of removing cellular receptors from the surface of target cells. Hence they are also known as "receptor destroying" enzymes. We have cloned and expressed several recombinant viral O-acetylesterases. These enzymes were secreted from Sf9 insect cells as chimeric proteins fused to eGFP. A purification scheme to isolate the recombinant O-acetylesterase of influenza C virus was developed. The recombinant enzymes derived from influenza C viruses specifically hydrolyze 9-O-acetylated sialic acids, while that of sialodacryoadenitis virus, a rat coronavirus related to mouse hepatitis virus, is specific for 4-O-acetylated sialic acid. The recombinant esterases were shown to specifically de-O-acetylate sialic acids on glycoconjugates. We have also expressed esterase knockout proteins of the influenza C virus hemagglutinin-esterase. The recombinant viral proteins can be used to unambiguously identify O-acetylated acids in a variety of assays.